In general, the present invention is directed to a polymer comprising amine groups dispersed throughout the polymer backbone which enable ionic movement for use in various applications. More specifically, the present invention is directed to a conductive polyamine-based polymer electrolyte, which may optionally be cross-linked and which is suitable for use in, for example, batteries, fuel cells, sensors, supercapacitors and electrochromic devices, as well as to methods for the preparation thereof.
Society continues to place more and more stringent demands on existing sources of energy, requiring that they be more efficient, more environmentally friendly, etc. For example, as developments in microelectronics continue to advance, the need for smaller, lighter, more powerful, and/or longer lasting energy sources increases. As a result, existing battery technologies have been stretched to their limits, requiring new technologies to be developed. Similarly, as environmental issues, such as pollution or emission controls and resource conservation, continue to be given more attention, alternative energy sources, such as the use of high energy density batteries or fuel cells, grow in importance.
Currently, efforts remained focused on the development of improved secondary, or rechargeable, cells, having high energy densities. While there are many different types of secondary cells, lithium ion cells are an area of emphasis because, as compared to most other systems, they possess longer lifetimes and higher capacities. Generally speaking, current state-of-the-art lithium cells, or batteries, consist of (i) a cathode, (ii) an anode, both of which are made of a material capable of intercalating/deintercalating lithium ions (the positive electrode intercalating during discharge and the negative electrode during charge), lithium being preferred because it is a high specific energy material, and (iii) an electrolyte. More specifically, current secondary lithium cells employ lithium metal or lithium ions in carbon as the anode, a chalcogenide salt (such as, for example, LixMn2O4, LixCoO2, LiV3O8 and LixNiO2) as the cathode, and a liquid or solid electrolyte. During discharge of a cell having lithium metal as the anode, lithium metal is oxidized into lithium ions at the anode, the ions then undergoing an intercalation reaction at the cathode; the reverse process at each electrode occurs during charging of the cell. For cells where both the anode and cathode are made of lithium ion intercalation materials, lithium metal is not involved and all redox processes occur in the intercalate matrix.
The composition or form of the electrolyte employed in these cells is of particular interest. For example, while many liquid electrolytes have high conductivities, their use is problematic because of concerns over leakage of hazardous materials and loss of performance resulting from drying due to evaporation and/or leakage. Solid electrolytes are particularly attractive since they offer new opportunities in design that are not available with liquid electrolytes. For example, solid polymeric electrolytes possessing elastomeric properties are attractive because they would be able to expand/contract within the cell to ensure continuous and full interfacial contact with the electrodes as volume changes within the cell occur in operation; additionally, such elastomeric properties would enable cells containing these electrolytes to be more easily fabricated. Another advantage of solid electrolytes is the fact that they would essentially eliminate concerns over leakage and drying problems experienced with liquid compositions. Finally, such electrolytes may be formed into thin films to minimize resistance and to reduce overall volume and weight of the cell.
Polymers which have been examined for use as solid electrolytes include those based upon linear chain polyethers, such as poly(ethylene oxide) (“PEO”) and poly(propylene oxide) (“PPO”), with associated alkali metal salts (see, e.g., Le Nest et al., Polymer Comm., 28, pp. 302–305 (1987); and, Tsuchida et al., Macromol., 88, pp. 96–100 (1988)). However, such electrolytes display conductivity in the range of practical use (e.g., σ=10−5 to 10−3 S/cm) only at temperatures well above room temperature. In addition to exhibiting poor conductivity, particularly at room temperature, acceptable physical properties are lacking as well; for example, these polymers typically form thin films which are tacky (the polymers at times being too close to the liquid state), brittle, or too heat sensitive. While many of the deficiencies in physical properties may be addressed by the addition of a plasticizer, poor conductivity remains an issue. Additionally, while some have proposed the use of gel electrolytes (such as those containing poly(vinylidene fluoride), or “PVdF”), these too have been found to exhibit insufficient conductivity at room temperature, and to lack desirable physical properties (see, e.g., U.S. Pat. No. 5,998,559). One very common problem with gel electrolytes, often formed by the addition of an electrolytic solution to a polymer matrix, is that they have problems with leakage or evaporation of the electrolyte solution, the matrix being unable to contain the electrolyte solution over time.
Due to its structural similarities to PEO and PPO, others have also investigated the use of poly(ethylenimine) (“PEI”), as well as various derivatives thereof, as a solid polymer electrolyte. (See, e.g., Yokomichi et al., U.S. Pat. No. 5,204,196; M. Watanabe et al., Macromol., 20, pp. 968–73 (1987); J. Paul et al., Electrochimica Acta, 37, pp. 1623–25 (1992); and, R. Tanaka et al., Solid State Ionics, 60, pp.119–23 (1993).) However, while various forms of PEI have been studied, including (i) linear PEI, (ii) a PEI polymer backbone with PEO side chains extending from the nitrogen atoms in the backbone, and (ii) branched PEI (including methylated PEI), a commercially viable PEI-based electrolyte has not been produced; that is, while various forms of PEI have been studied, as yet no form has been identified as an electrolyte having both electrical and mechanical properties making it suitable for use in commercial applications.
In view of the foregoing, it can be seen that a need continues to exist for a solid polymer electrolyte suitable for use in a number of commercial applications. Ideally, the electrolyte would possess high conductivity while having superior mechanical properties (i.e., strength, elasticity, electrochemical stability, etc.) for the desired application. In addition, such an electrolyte would be easily and inexpensively manufactured, and capable of being formed, extruded, etc. into any of a number of desired shapes or forms.